I've been on both sides of this - hiring, and being the one interviewing.
My situation was a little different - I was also 20 weeks when I interviewed (and got hired), but it was for a summer job, something that could "potentially" be ending right about the time I was due, so the timing worked perfectly.
In that situation, I did not disclose. I struggled with that thought, but in the end, knowing the timing of the 4 month position, I decided that they should hire me for ME, not taking the growing belly into account. It was also front-desk at a community center, where I thought the belly wouldn't be an issue.
It wasn't. They hired me, and I disclosed during THAT conversation, oh by the way, I am pregnant; I am due X... does that change your opinion? haha.
Granted... there was silence for a moment. This was certainly a surprise to the manager. But in the end, they decided the same thing - the pregnancy wasn't an issue.
On the flip-side, I was hiring for an assistant a few years back. It was a FT position. And a very wonderful, qualified, person applied. And was very open about her pregnancy.
It seemed discriminatory to not hire her on the basis that she was pregnant. However, because I needed someone with some longevity, the idea of maternity leave (even for two months) was more than our company could deal with. It was a very hard call. But, at the time, and not being a parent yet, I summed it up with someone saying - oh, by the way, I also have a cruise scheduled for three weeks.... I'll be back and in great form, but I will need the scheduled time off. And in that case, I wouldn't have hired that person either.
So... IF you choose to disclose, which if it's a FT position, I think you probably should.... you should definitely make a case for your long-term plans. You plan to return to work after X weeks. This has already been decided by you & family, etc., and how you absolutely plan on making this happen (even if you change your mind as time goes on). Something to ensure that this is a job you're interested in and (gulp), baby is not going to alter that. (that's just a horrible way to put it... but...), you are applying for a job. That's really all the company cares about.
Good luck!!!